Discovering hippocampal connections responsible for episodic memory

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The hippocampus is the region of the brain that is responsible for episodic memory. For decades, neuroscientists have been mapping the hippocampus’s neural circuits to better understand how memories are stored, retrieved, and lost.

The trisynaptic circuit, discovered by the legendary anatomist Santiago Ramón y Cajal more than 100 years ago, has long been considered the anatomical substrate responsible for learning and memory. But the trisynaptic circuit involved only the entorhinal cortex and the dentate gyrus, CA1 region and CA3 region of the hippocampus; the tiny CA2 region of the hippocampus, located between CA1 and CA3, was not thought to play a critical role.

When a 2005 study using molecular cell markers revealed that the CA2 region might be wider than previously thought, neuroscientists began to wonder whether CA2 had a role in memory. To address this, scientists needed to know more about the CA2 region’s boundaries and its neural connections to CA1 and CA3. But traditional methodologies like dye injections and electrophysiological stimulation of axon bundles haven’t been up to the task.

Now, MIT neuroscientists, applying state-of-the-art single neuron electrophysiological and optogenetic technologies, have succeeded in laying out the precise map of neural circuits involving the CA2 region.

The discovery was made by researchers working in the laboratory of Nobel laureate Susumu Tonegawa, a principal investigator at MIT’s Picower Institute for Learning and Memory, professor of biology and neuroscience, and director of the RIKEN-MIT Center for Neural Circuit Genetics at MIT and the RIKEN Brain Science Institute in Japan. Authors of the study are Keigo Kohara, Michele Pignatelli, Alexander Rivest, Hae-Yoon Jung, Takashi Kitamura, Junghyup Suh, Dominic Frank, Koichiro Kajikawa, and Tonegawa. Their findings appeared in the Dec. 15 issue of Nature Neuroscience.

Advanced technologies yield breakthrough discovery

The scientists used multiple molecular methods and a series of tools and electrophysiological experiments to decipher neural circuits associated with the CA2 region. Using cell-type-specific tools, the scientists infected only CA2 cells with a virus that made them express a light-sensitive protein.

Exposing the infected cells to a certain color of light causes them to emit nerve impulses. Using this optogenetic technology, the MIT researchers revealed synaptic connections between the dentate gyrus and CA2 regions that had been completely unsuspected. Conversely, the researchers demonstrated that the direct connections from the entorhinal cortex layer III to CA2 that had been claimed by the traditional method actually do not exist. These discoveries led the researchers to conclude that the CA2 region is home to a new trisynaptic circuit that runs in parallel to the classic trisynaptic circuit. These findings will force major changes in brain anatomy textbooks.

Novel circuit enhances understanding

“The identification of this extremely precise circuit design is poised to drive deeper understanding of the biology of learning and memory,” says co-author Kohara. In addition to making highly useful methodological contributions to significant ongoing brain research initiatives — including the European Union's 1 billion-euro Human Brain Project, the United States' $2 billion BRAIN project, and the Microsoft-driven $300 million Allen Brain Atlas — the research is expected to facilitate investigation of fundamental alterations linked to pathological conditions associated with amnesia, including Alzheimer’s and schizophrenia.

The research was supported by grants from the National Institutes of Health, the Japan Society for the Promotion of Science, and RIKEN Brain Science Institute.